Building block



FIG. 1 is a top plan view of our building block showing our new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a rear plan view thereof;

FIG. 4 is a bottom plan view thereof;

FIG. 5 is a left side view thereof; and,

FIG. 6 is a right side view thereof. 

The ornamental design for a building block, as shown and described. 